Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface, showing our new design; and,

FIG. 2 is an enlarged view of the graphical user interface within the display screen or portion thereof in FIG. 1.

The broken lines illustrate environmental subject matter and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 